Crystal filters have been provided having different frequency response characteristics, such as a bandpass response, as required for various different applications. Although such crystal filters can provide very sharp selectivity, the crystal may have spurious responses which result in unacceptable frequency attenuation levels in the reject region of the filter. In an attempt to overcome this problem, additional crystals may be added to the filter. However, this makes the crystal filter design much more complex and may produce other problems.
Bandpass crystal filters have been provided using dual coupled monolithic crystal resonators, but such filters have not had sufficient attenuation at frequencies adjacent the filter passband for use in some applications. To provide a filter of a different known design, with additional crystal elements to overcome the problems, would result in a complex and expensive crystal filter configuration.